Weapons of mass destruction vs. Nuclear Testing

When America was supposedly attacked on 9/11 by a bunch of Muslim extremist terrorists the CIA was quick to disclose the 15 persons responsible for the attacks.

The ability to track down and find 15 terrorists, some of whom were dead, in a matter of days is quite an achievement and a clear indication of the superiority of America’s intelligence agencies.

It was later discovered that:
“There can be no doubt that Iraq has continued to pursue its goal of obtaining weapons of mass destruction.”

Despite the fact that Afghanistan really has absolutely nothing to do with Iraq somehow the idea of invading Iraq won support and the invasion went ahead.

Now, after the invasion had taken place they found no traces of these weapons of mass destruction? What happened to their amazing intelligence capabilities that allow them to find 15 people responsible for 9/11 in just days?

Do you mean to tell me that this amazing agency with all their sources and limitless resources could not track down dirty great weapons of mass destruction in 5 years, but had no trouble tracking down 15 terrorists in just 5 days?

Furthermore, if Iraq was really made out to be the rouge state that it was why would the US risk an invasion against them. And why did Iraq not use these ‘weapons of mass destruction’ against the invading forces?

Finally we come to present day. Now it seems evident that the US policy on terrorism and the development of weapons of mass destruction is to invade the offending country on sight. This makes no military sense of course, but evidently this is their policy as clearly seen with the entire Iraq affair.

So why is it that North Korea, with their nuclear facilities and clear intentions to make nuclear weapons and reportedly successful two nuclear weapons tests, have not been invaded by the US? Doesn’t North Korea violate their policy on terrorism in a much more alarming and immediate way than Iraq ever did?

In fact the US is playing a completely different tune. They don’t want to invade on the chance of starting a nuclear war.

How is this scenario different to the Iraq scenario?
How does invading a country to stop the country starting a war even make sense?